In general, the machine structural components such as gears, shafts, pulleys, constant velocity universal joints and the like used for a variety of gear transmission devices to begin with a transmission and a differential gear for an automobile, as well as crank shafts, con'rods and the like are obtained by subjecting a steel for machine structural use to forging and the like and thereafter being finished into the final shape (the shape of the component) by cutting work. Because the cost required for the cutting work occupies a large portion in the total manufacturing cost, the steel for machine structural use is required to be excellent in machinability.
Also, it is desired that the machine structural component is excellent in fatigue property (particularly in pitting resistance). Therefore, the machine structural component is manufactured by finishing to the final shape (the shape of the component) by cutting work, and being subjected thereafter to a surface hardening treatment such as a carburizing treatment, a carobonitriding treatment and the like (including in the atmospheric pressure, low pressure, vacuum, and plasma atmosphere) in order to improve the fatigue property.
In cutting work for manufacturing a gear in particular out of the machine structural components, it is common to perform gear cutting by a hob, and cutting in the case is called interrupted cutting. As a tool used for bobbing, high speed tool steels coated with AlTiN and the like (may be hereinafter abbreviated as a “high-speed tool”) are most popular at present. However, gear cutting by hobbing (interrupted cutting) using a high-speed tool is performed at a low speed (approximately 150 m/min or below cutting speed specifically) and at a low temperature (approximately 200-600° C. specifically), but the tool is likely to be brought in contact with the air because of the interrupted cutting, and there is a harmful effect that the tool becomes liable to be oxidized and worn. Therefore, in the steel for machine structural use used for low speed interrupted cutting such as hobbing and the like, it is required to extend the tool life in particular out of the machinability.
As a technology improving interrupted cutting property, in the patent literature 1, a steel for interrupted high speed cutting containing Al: 0.04-0.20%, O: 0.0030% or less is proposed. According to the technology, by subjecting the steel with increased Al content to interrupted cutting at a high speed, Al oxide is adhered on the tool surface, and thereby the tool life is improved. However, in the steel for interrupted high speed cutting, high speed interrupted cutting with 200 m/min or above cutting speed is usually in mind, and low speed interrupted cutting such as hobbing is not intended.
On the other hand, as a tool used for cutting work, in addition to the high-speed tool, there is also a tool subjecting cemented carbide to coating of AlTiN and the like (may be hereinafter abbreviated as a “cemented carbide tool”). Because of the problem that “chipping” is liable to occur when applied to a normalized material, the cemented carbide tool is usually applied to continuous cutting such as lathe cutting and the like. The continuous cutting such as lathe cutting and the like is normally performed at a cutting speed exceeding 150 m/min, and is performed at a high speed of 200 m/min or above in many cases.
Thus, the cutting mechanism is different between the interrupted cutting and continuous cutting, and a tool matching each cutting is selected. However, it is demanded that the steel for machine structural use as a material to be machined exerts excellent machinability in both types of cutting.
In the meantime, after being finished into the final shape, the steel for machine structural use is subjected to a surface hardening treatment such as a carburizing treatment, a carobonitriding treatment and the like (including in the atmospheric pressure, low pressure, vacuum, and plasma atmosphere), is further subjected to the heat treatments such as quenching and tempering as well as induction hardening and the like, and is strengthened to a predetermined strength. However, when it is subjected to a thermal effect, the toughness may drop and the impact performance may deteriorate.
As a technology improving the impact performance, in the patent literature 2, a steel for machine structural use containing Al in a range exceeding 0.1% and 0.3% or below is proposed. In the document, it is disclosed that the machinability and the impact performance can be improved by reducing the solid-resolved N amount, and that the cutting performance effective in a wide cutting speed range from a low speed to a high speed can be obtained by securing a proper amount of solid-resolved Al and AlN effective in improving the machinability by optimizing the Al content. According to the document, the impact performance of the steel for machine structural use is evaluated by measuring absorbed energy by the Charpy impact test. However, the absorbed energy achieved in the document does not reach 50 J/cm2, and further improvement of the impact performance is required.
In the patent literature 3, the present applicant also proposed a steel for machine structural use exerting excellent machinability in both of interrupted cutting by a high speed tool and continuous cutting by a cemented carbide tool and exhibiting excellent impact performance even when it is subjected to carburizing-oil quenching and thereafter is subjected to a tempering treatment. According to the technology, the machinability and the impact performance are improved by properly controlling the contents of Cr and Al as well as the ratio of the contents thereof.
Also, as described above, it is also desirable that the machine structural components subjected to a surface hardening treatment such as a carburizing treatment, a carobonitriding treatment and the like after finished into a final shape are excellent in fatigue property (pitting resistance in particular).
As a technology providing a case hardened steel subjected to a surface hardening treatment, the patent literature 4 is known. According to the technology, the precipitation amount of AlN after hot rolling is limited to 0.01% or below, and, in order to prevent coarsening of the grains in carburizing, AlN and NbN are not utilized as the pinning particles, but Ti-based precipitates mainly of TiC and TiCS are utilized. Also, in order to improve the fatigue property (rolling fatigue property in the document), the maximum size of the Ti precipitates is made small. However, according to the technology, the Al content is stipulated in a range of a small amount of 0.005-0.05%, and it is not the technology improving the fatigue property of the case hardened components containing Al in a range of 0.1% or above.